1. Field of the Invention
The present invention relates to a ferromagnetic tunnel junction element to sense a magnetic field and relates also to a device provided therewith such as tunnel magnetoresistive head, magnetic head slider, and magnetic disk drive.
2. Description of the Related Art
High-density magnetic recording needs a sophisticated read head. This requirement is met by the magnetoresistive head based on giant magnetoresistance effect (GMR effect), which has recently gained commercial acceptance. The current magnetoresistive head produces its GMR effect through the spin valve film-composed of ferromagnetic layer, non-magnetic metal layer, and ferromagnetic layer. The spin valve film of this structure has a limited magnetoresistance ratio of about 10%. Hence, a more sensitive magnetoresistive element is required.
Researches to meet this requirement have recently revealed a phenomenon called ferromagnetic tunnel effect. This tunnel effect manifests itself in the junction structure composed of ferromagnetic layer, amorphous insulating layer, and ferromagnetic layer, and it depends on the relative angle of magnetization of the two ferromagnetic layers. (J. Magn. Mater. 139, L231 (1995); Phys. Rev. Lett. 74, 3273 (1995)) Because of its ability to give a magnetoresistance ratio greater than 10% at room temperature, the above-mentioned phenomenon has attracted keen attention on research and development for new magnetoresistive elements. For its extremely high sensitivity to magnetic field, the ferromagnetic tunnel effect film is greatly expected to find use as a read head for ultra-high-density magnetic recording exceeding 100 Gbit/inch2.
The ferromagnetic tunnel effect had been analyzed by Julliere's theory (Phys. Lett. A54, 252 (1975)), which is useful particularly for the system containing an amorphous insulating layer. This theory defines the spin polarization as P=(D↑−D↓)/(D↑+D↓), where D↑ and D↓ denote respectively the density of state (or the number of states per unit energy) of up-spin and down-spin in the ferromagnetic layer at Fermi level. The value of P is used to represent the magnetoresistance ratio at zero voltage as follows.TMR=100×2P2/(1−P2)[%]
The foregoing expression indicates that the larger the value of P, the higher the magnetoresistance ratio. This idea has stimulated the research on the tunnel junction with La0.7Sr0.3MnO3 which is a half-metallic ferromagnetic substance having P=1 at the absolute zero (Phys. Rev. Lett., 82, 4288 (1999)) This half-metallic ferromagnetic substance is regarded as an effective spin injector in view of the fact that only up-spin electrons contribute to conduction. This substance is under the stage of basic research and its practical application is being pursued.